Arrangement for transmitting electric signals occupying a wide frequency band over narrow band circuits



Nov. 27, 1951 s. s. H|l v 2,576,115

ARRANCEMENT ECR TRANSMIITINC ELECTRIC sICNALs CCCURYINC A wIDE FREQUENCY BAND CvER NARRow BAND CIRCUITS Filed Feb. 4, 1948 zzwru Emma uwlu @wais www@ Patented Nov. 27, 1951 ARRANGEMENT FOR TRANSMITTING ELEC- TRIC SIGNALS OCCUPYING A WIDE FRE- QUENCY BAND OVERI NARROW BAND CIR- CUITS i Stuart Seymour Hill, London, England, assigner to International Standard Electric Corporation, New York, N. Y.,

a corporation of Dela- Application February 4, 1948, Serial No. 6,316 In Great Britain February 7, 1947 (Cl. 17H4) 13 Claims.`

The present invention relates to electric signal transmission systems of the kind in which signals occupying a relative wide frequency band are transmitted over two or more circuits each of which is capable of transmitting only a relatively narrow frequency band.

The invention has its principal application to broadcasting circuits.

It is frequently necessary to send broadcast material to the studios by telephone line. Sometimes special broadcast channels designed for a wide frequency band with low noise and distortion are available, or ordinary open wire lines can perhaps be specially equalised for a given occasion. Very often, however, only narrow band commercial telephone channels will be available. Such channels may, for example, pass a band from about 300 to 3,000 cycles per second, while the broadcast band which it is desired to transmit may extend from about 100 to 6,000 cycles per second.

It has therefore already been proposed to split the wide broadcast band into two sections, which after the necessary frequency shifts are transmitted separately over two different commercial COmbIling of the two parts of the band depend on this band-width. In other words, if the circuits are designed for telephone channels with a given band-width, they cannot be satisfactorily used with channels having a different bandwidth.

. It is therefore the principal object of the present invention to overcome this difficulty, and to provide an arrangement which can be easily adapted for use with telephone channels of various band-widths.

This object is achieved according to the invention by providing an arrangement for transmitting an electric signal occupying a relatively wide frequency band over a plurality of communication channels each of which is capable of passing only a relatively narrow `frequency band, comprising means for dividing the signal band into a plurality of sections of widths corresponding respectively to the widths of the bands passed by the said channels, means for frequency shifting each of one or more sections to the frequency range.

passed by the corresponding channel and meansl for transmitting the frequency shifted sections. over the respective channels, characterised in this, that the frequency shifting means in thev case of at least one section comprises two successive frequency changes, one of which employs a fixed carrier frequency and the other a carrier frequency which is adjusted to a value determined by the cut-off frequency of the corresponding channel. j

The invention will be described with reference to the accompanying drawings, Figs. 1 and 2 ofl which respectively show block schematic circuit diagrams of the transmitting and receiving arrangements of a system according to the invention. Terminals l and 2 of Fig. 1 are intended to be connected respectively to terminals 3 and 4 of Fig. 2 by separate telephone or other relatively narrow band circuits or lines. The system to be described will be assumed to be designed to meet a particular series of requirements, but4 it should be understood that it may be adapted for other requirements. None of the frequency ranges given below are intended to set limitations to the system, and are only `quoted for the sake of example, and may be extended Without any dimculty.

It will be assumed that the broadcast band which it is desired to cover extends from 100 to 6,000 cycles per second, and that the commercial telephone channels cover a band from about 300 tow'3,000 cycles per second. It will further be` assumed that the commercial telephone circuits likely to be available may have upper cut-oli? frequencies which may lie anywhere between about 2,700 and 3,200 cycles per second. The upper cut-off frequency of the commercial circuits will be denoted by fc.

At the transmitting end of the system the programme signals covering the wide band are amplified in a preamplifier 5 ,and are then passed through a clean-up low pass filter 6 which removes those upper frequencies which the system cannot be expected to transmit. This filter will therefore limit the band betweeny and about 6,300 cycles per second. The limited band of frequencies is then supplied in parallel to upper and lower channels through a conventional hyb rid `coil network l, the place of the balancing impedance being taken by a synchronising oscillator 8 to be described later. This hybrid coil network prevents appreciable interaction between the two channels. The band is then applied in the lower channel through'the attenuating pad I Vto a modulator l supplied with a carrier wave of frequency 12 kc./s. from an oscillator Il, the

lower sideband extending from 5.7 Jtou11191 lic/s.M

being selectedflbyaltef lf2 which also iatte'nuatesj frequencies up to about7.5 kc./s. foi1` the purpose of minimising programme leak through the modulator. The pad 9 may introduce an attenuai tion of about 12 decibels, andiisprovid'ed: inorder ,channel has therefore been to remove that porv"tion of the original programme band which is dealt with bythe-lower channel, and to shift the to ensure that both channels? are .operated at'fY about the same level. the middle and lower frequencies predominate,

In the programmeband` and so without the pad 9 thelflower-,channel would be at a much higher level thanthenpperchannel Y,

and there would then be dangerofzthexintroduction of excessive noise into the upper channel.

The lower sideband from thefilter l2 is .applied to a second modulator s liedw'ith al-.c'arrier upp l "programme leak, and delivers the lower sideband wave of 12.2 kc./s. from an oscillator I4 the output of .which is applied through an amplifier 2.15V

appears asl-1.300 cycles-in the Jfinally* transformed band. The highest frequencyof` the originalwide input-:band f which1is passed through thelower. channel is A3,kc./s.,f `and thisappears as 9 .kc./S. after. thefi-lrst modulation; 1and.3.2l c./s. .after 'the second. j(Frequencies.above 3.2.-kc./s. will be vattenuated by the commercial channel,y though there v mayebe some.. transmission upto about 4.7 ks./s., correspondingv to.4.5kc./s..in the original input -ba/nd. Thus there `isscme transmis sionon the frequencies between 3.l and 4.5 lcd/s. ofthe signal band. `Howeverfany.frequencies in the range 3.2110 4.7 kc./s. which may. be actually transmitted .by the-commercial .channel are ,further. attenuated .at a klater .stagerr in the receiving terminal. Y. It wlilllbeseen thatthe eifectof the. two modulations is toraise. all the.frequenciesl in the lower part,.of .the'band by v200 .cycles per second (the difference. between .the frequencies of the two o sc'illators'L. Frequencies in theupper part of. the band have been partlylremovedbyl the line and partly ,by the high pass section ofthe filter I2 between the two modulators'll) and'l3. f

The modulation process in the upperv channel will now be explained.' The .original wide input programme band isapplied from the hybrid coil network' 1 to aV rst modulator' l5 supplied with carrienwaves from an oscillator Hof adjustable frequency. If fc is the cut-off frequency of the telephone channels, then 'this oscillator should beset 'to aV frequency of`11.51fc"kc./s. In the particular case chosenin which f'c is 3.2 kc./s., the

frequency supplied to the i'lrst modulator i6 should therefore be 14.7 kc./s. The lower side band extends fromv8.4 to 1.4.6 kc./s., and is passed through a lter le having apass bandV extending from 8.8 to y11.7 isc/s., which-removes the upper side band, andin addition attenuates frequencies up -to 6 kc./s., that is those frequencies which mightappear. owing tov .programme leak. The portiongfofY this lowerlside band .passed by thelter lisfap'plied toa 'second'modulator I9 supplied with a carrier wave of .frequency of 12. kc./s. from the oscillator! l which supplies the first modula- .tor I i! .in the lower. channel. The lower sideband, coveringthe range '300 td 3,200 cycles per second, is .supplied throughan amplifier` Y29' to the secondltelephonejchannel connected Ato terminal.|. Frequencies between 6 and l8.8 kc./s., which yare not much "attenuated by the lter i8 Yupper-ffrequencies ofthe band downwards by 2.7 kc/s., the diierencebetween the frequencies of the two oscillators.

1 At the receiving terminal shown in Fig. 2, the

' Aband ofjfrequencies of the lower channel which toA a.second demodulator 24 supplied with a carrier frequency from an oscillator 25 at 9.15 lio/s. The lower sideband from the demodulator. 2K coverstherange lll'to 3,600'cycles persecond of the original signal band.. Besides the unwanted upper sideband (a) which covers the Vrange 15 to 1749 kcJ/s.; there will be asm'all progra-mmev leal; (b) inthe range' 5.85 to 8;75 koi/sand also those frequencies (c). immediately above 3.2 kc./s.

which have not been effectively removed bythe filters 6 and l2. 4 Y

Thefrequencies (a) and (b) are-removed by a suitable low pass filter 26 in the output circuit in which the bands from! the lower and upper channels arerecombined. The frequencies (c) may beat vwithV frequencies` legitimately transmitted over the upper channel. simple lowpass lter section 21is therefore includedin the lower branch after the second demodulatorZll for the purpose of attenuating these frequencies sufciently to'render them innocuous. The filter 21 also'preventslintermodulations bctweenthel high frequency Apro uct-s which may pass between the upper and lower channels on account ofthe imperfections of the hybrid coil network l2 3 which', as` willv be Y explained presently,

is: used to combine the two parts of' the pro-` gramme band at the receiving end. j

'The' upper frequency band-300 cycles to 3.290 kcl/slwhich: farrives over `the' `'line connected to terminal 3 is shifted tothe'lrange 3 to 5.9 koi/s. by two' successive demoolulations.'V The first demodulator'z is sup-plied with carrier waves at a frequency of 9.15 kc;/s. fromthe'oscillator 25 which supplies the demodulator 24 in the lower channel. The lower sideband coveringthe range 5.8to 8.65

kc./s. is selected by the band pass filter i yand delivered to the second demodulator 3! supplied fromv an adjustable oscillator 32 providing carrier waves at a frequency of'8. 65-fc kc./s., which for'the present case irl-whichV JcI is 3.2 lic/s. will be 1-l585'kc./s.

The two bands to 3,090 cycles and 35000 to '5,900 cycles obtained' respectively from the filter 21 and demodulatorl are re-united by the hybrid coil network 28 already referred to, arranged so that the upper andV lower channelsare conjugate, so that intermodulation between these channels c is substantially prevented. An attenuating pad 32' corresponding to the pad 9 and introducing the same attenuation (e.` g. 12 decibels) 'is introduced 'initheupper channel between the demodulation 3l `and the hybrid coil network V28 "to restoreithe 'two sections of the programme band to th'eproper relative levels.

An attenuatingpad 33 introducing for example about 6.5 decibels maybe included if desired in An additional the common output channel between the hybrid coil network 28 and the filter 26 already mentioned, and the programme band may be delivered -to the output conductor 34 through a suitable amplifier 35. It will `be understood thatthe adjustable oscillators l'l and 32 supplying the first modulator lli` at the transmitting end, Fig. 1 and the second demodulator 3l at the receiving end Fig. 2 of the upper channel will be set to provide frequencies determined by the cut oiffrequency of the telephone channels which will be used. If for example this cut olf frequency is only 2.7 kc./s.,.the frequencies of theoscillators will be respectively 14.2 and 11.35 kc./s. and the programme band which can be transmitted over the two telephone channels will be correspondingly reduced tothe range 100 to 4,900 cycles per second.

Besides adjusting the oscillators in accordance with the cut-off frequency of the telephone channels it will also be necessary to adjust the filter 2,1 (Fig. 2) used for preventing intermodulation between the upper and lower channels. As this filter can bequite simple this will cause no difficulty. i The oscillators used to supply the modulators should be of a very stable and accurate type, and in order to avoid dis-tortion it is necessary to ensure that the oscillators at the receiving end Fig. 2 are `correctly set so that the overall tone translation effect is zero Within a tolerance of perhaps i5 cycles. For this purpose, the oscillator `8 (Fig. 1) is provided for transmitting over the system a complex wave at a frequency of, say, l kc./s. with prominent 2nd and 4th harmonics. The fundamental and second harmonic pass over the lower branch, and the fourth harmonic passes over the upper branch. The received complex tone after recombination at the receiver (Fig. 2) is applied to a monitoring device 36 connected to the output of the amplifier 35, which device includes selective circuits which` pick out the three frequencies and a small oscillograph. The three frequencies are applied in pairsto the deflecting plates of the oscillograph, and a stationary figure will be obtained only when the tone translation effect is zero.` The oscillators at the receiving end are slightly adjusted to produce this result.

If desired, ringing and telephone circuits 31 and 38 may be provided in the upper branch at thetransmitting and receiving ends, between the telephone channel and the modulating or demodulating equipment in each case. l These circuits permit the operators to communicate with one another when making the initial arrangements for the broadcast. `These circuits are of known type and should preferably be designed to render ineffective the broadcast equipment of the upper channel during ringing and conversation by the operators. By employing the `upper channel conversation would be possible Without interrupting a broadcast, the quality `of which would, however, be temporarily impaired by removal of the upper part of the band.

` The frequencies chosen for the oscillators ll and 25 of Figs. 1 and 2 are not very critical, and other values could be used. The frequency 12 kc./s. `for the oscillator Il was selected to be about double the maximum frequency of the broadcast band to be transmitted,` in order to minimise programme leak. The frequency 12.2 kcJ/s. for the oscillator I4 then follows since a frequency shift of 200 cycles per second is required for the lower part of the band. Y The frequency 11.5-l-fc for the oscillator I1 is determined in the following l way. The width of 6i' the `band passed bythe commercial circuits `is fc-D kc./s. and therefore the broadcast band must be divided at the frequency s Hence the maximum frequency of the upperportion `of the broadcast band which can be transmitted over the upper channel is Zic-0.5 kc./s. This has to be transferred by the double modulation to fc and so since the oscillator Il is used to supply a frequency of `12 kc./s. to the modulator I9, it follows that the frequency of the oscillator I1 must be 12|-fc-0.5=11.5|fc kc./s.

lThe derivation of the fixed frequency portion of the frequency of the oscillator Il may be further explained as follows:

Let fb=the frequency defining the lower limit of the Wide-broadcast band.

Let fc=the upper cut-oif frequency of the com--` mercial channel. i i

Let fl=the frequency defining the lower limit of the commercial channel. z'

Let 02=the frequency of the-second oscillator of the upper channel.

Let X=a fixed frequency.

` Then i The derivation may be as follows:

Now

The frequency of the first oscillator-the lfre-` quency defining the upper limit `of the Wide-,- broadcast band=intermediate frequency.

` And:

The frequency of the second oscillator-the in termediate frequency=fc In this equation if l2 kc. is substituted for 02, .1 kc. for fb, and .6 kc. for 231, X wil1=11.5, the

value of the fixed frequency indicated.

The frequency for the oscillator 25 (Fig. 2) .is also ,determined by programme leak considerations. It can be shown that to avoid programme leak in the demodulator 3l, the frequency of the oscillator 25 should be greater than Bo-0.5 kc./-s., and therefore, in the case chosen, in which the maximum value of fc is 3.2 kc./s. the minimum value for the frequency of the oscillator 25 will be 9.1 kc./s. A larger value than this could be used, and the value actually chosen, namely 9.15-

kc./s. was selected to suit a particular filter which was available. The frequencies of 9.35 kc./s. for the oscillator 22 and 8.65-l-fc kc./s. for the oscillator 32 follow from the value 9.l5kc./s. se-` Y 1 l''cted'foir theroscillatcr necessaryishifts .to" restore`A the sections' of the broadcast band totlieir'proper positions. Y

It will beclear that a broadcast band could be split intoY more than two sections for separate transmission 'overa similar numberv 0f" differ-i entcominercial circuits, means'being provided according to similar principles for conveniently :adjusting the frequency changing arrangements i'rifaccordance with the pass band ofthe commercialfciruts.` While the-arrangement has been pri-niarilyappledto the transmission of broadcast `progranime signals', it could be used for' transfnitting any typeof Signals occupying a wide band over twoor more' narrowl band circuits, for'any desired purpose. What is'claimd is:

1. An electric signalreceiving arrangement for rei'vi-ng signals having frequencies lying within a wide band and which are transmitted over a plurality of channels each having a signalV pass band narrower than said wide band whereinlsig-L' nalshaving" frequencies within at least one band portion of said wide band, which portion' is adjustable with respect to the pass-band-charac- Y teristics of said channels, are converted to aband cived signals back to the corresponding band portion of said Wideband, means'for receiving signals transmittedV over said one channel,- means including a first oscillator adjusted to oscillate aannemer-.tc obtain me@ andere at a predetermined frequencyV for restoring said,

last mentioned signals back to said one band of Said wide band, said last-mentioned means comprising means' for converting said last-mentioned signalsfto aV4 new band of frequencies, said converting means including a second oscillator vadjusted to oscillate at a frequency equal-tothe sum of the cut-olf frequency of said one channel plus a fixed frequency which is a function ofthe frequency of oscillation of said rst oscillator and the frequencies defining the lower limits of said wide band of frequencies and the pass band of said one channel, and means for converting the signals having frequencies within said new band to signals having frequencies within said one band portion, and combining means connected to the output of each of said receiving and restoring means to reform said wide band of frequencies. i

2. A broad"bad electric signal Vtran'emission system including a transmitting arrangement'for transmitting an electric signal occupying a relatively wide frequency band over aplurahty of communication channels each of which is capable of passing' only 'a relativelynarrow fren quency'band, ysaid arrangement being adjustable for use with channels having different pass-band characteristics, comprising means for dividing the signal band into' a plurality of sections of widths corresponding respectively to the vidths of the bands passed by said chan-nels, means'ifor frequency shiftingat-least one of Said sections to the frequency range passed by the corresponding channel, means for transmitting the frequency shifted section overl the correspondingchannel, th'efrequenc'y shifting' means for said one section comprising two successive frequency changngstages, one of said stages havingl means for generating a fixed carrier frequency, the other stage-having means Vforfgeneratinig a carrier free cur-*off frequency lof said.:wirresponding: channel, the-fixed carrier frequency and thefrequencies ydefining?thev lower''limits of fthe "wide-bandol frequencies and-the passband'of the corresponding channel. Y broadband electric signall transmission system according to' claim 2 further comprising areceivingarrangement connected to said channels Vcomprising means for frequency shifting eachof said vsections back to the corresponding frequency range `of said wide frequency band, and means for "combining the restored sections to reform-'saidwide frequency band, said frequency shifting means at'the receiver for said one sece tion comprising A' two'A successive frequency chang-V ingj'stages,'fone"of said stages having meansrffor generating'abixed carrier frequency, the Aother stage'having means for generating a carrier free quency adjustable to a value determined by said cut-off frequency of said corresponding channel. 4. rsystemV according to claim 2 in which said di'vidingmeans is adapted 4to divide said'wide'frequency band intoftwo sectionsffor transmissionv over'two Anarrow band channels.' said frequency shifting means comprising Vmeans including two frequency changing stages for shifting the. lower frequency "section upwards to occupyithe band, passed by "one channel and means including two frequencychanging stagesfor shifting the upper frequency section downwards to occupy theband passed by the other channel.

15. A system according to claim 4 in which both of the two Vlast-mentioned frequency shifting means for the two channels include common means for generating a common xed carrier fre- OlU-BIICY- Y' 6. A system according to claim 2 in which theV signals `occupying the wide band are coupled to said frequency shifting means througha hybrid coil network.

7. A system/according to claim 2 further comprising a reeciving .arrangement connected to .said two narrow band channels comprising two frequency changing stages for shifting back .the

lower frequency section to the frequency range originally occupied thereby, and twofurther frequency changing ,stages for .shifting back the upper frequencyV section to the frequency range originallyV occupied. thereby, in which the two further frequency changing.. stages have means for generatingv respectively carrier frequencies one of which is fixed andthe other adjustable to a' value'V Adetermined `by the upper cut-off frequency and in4 which the first mentionedV frequency changing stages .both have means for generating xed carrier frequencies,V the carrier generating means `of oneV of said rst mentioned frequency changing stages constituting the xed frequency carrier generating means ,of one of thefurther frequencychanging stages. e

8'.A system `according to claim 7 in which .the transmitting arrangement includes means for attenuating the frequencies `of the lower section before transmission over the vcorresponding channel to equalizeV the levels of the bands transmitted over the two channels,and in which the receiving arrangement includes means for attenu-V ating the frequencies of the upper section by the same amount for -restoring the-relatveflevelsfof quency adjustable to aiyalue determined 'bythe u ing stages, ,whereby :thegtwo sections,;.of;.the wide band are recombined after frequency restoration, and means for connecting the output of said network to a common output channel.

10. A system according to claim 2 further comprising a receiving arrangement including means responsive to frequencies within said two sections for comparing the overall frequency shifts produced by the transmission of the two sections respectively over the said channels.

11. A system according to claim 2 in which the said comparing means comprises an oscillator connected to the input of said frequency shifting means in the transmitting arrangement and adapted to supply a given fundamental frequency accompanied by strong second `and fourth harmonics chosen so that the fundamental and second harmonic lie in the band occupied by the lower section, and so that the fourth harmonic lies in the other band, said receiving arrangement including a monitoring device adapted to compare the frequencies of the said fundamental frequency and harmonics as received.

12. An electric signal transmitting arrangement for transmitting signals having frequencies lying within a wide band over a plurality of channels each having a signal pass-band narrower than said wide band, said arrangement being adjustable for use with channels having different pass-band characteristics, comprising means for selecting and transmitting signals having frequencies within a rst portion of said wide band over a first of said channels, and means for transmitting signals having frequencies within a secand portion of said wide band over a second of said channels, said last-mentioned means comprising a rst oscillator for converting said lastmentioned signals to a new band of frequencies, and means including a second oscillator and a pass-band lter for converting the signals having frequencies within said new band of frequencies to frequencies passed by said second channel, said first oscillator being adjusted to oscillate at a frequency equal to the sum of the cut-off frequency of said second channel and a fixed frequency which is a function of the frequency of said second oscillator, the frequency defining the lower limit of said wide band of frequencies, and the frequency defining the lower limit of the signal pass-band of said second channel, whereby the limits of the second portion of said wide band will be determined by the pass-band characteristics of said second channel.

13. An electric signal transmitting arrangement according to claim 12 in which the fixed frequency portion of the frequency' of said first oscillator is equal to the frequency of said second oscillator plus the frequency defining the lower limit of the wide band of signals minus twice the frequency defining the lower limit of the signal pass band of the second channel.

STUART SEYMOUR HILL.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,595,135 Affel Aug. 10, 1926 1,812,405 Ives June 30, 1931 2,407,213 Tuniek Sept. 3, 1946 

